Making balls by metal spinning

Metal spinning is a technique where a flat metal disk is
pressed against a form while turning in a lathe. It is a
convenient method for the making of balls and other round
elements for use in high-voltage instruments. Below I describe
how I use this technique to make terminal balls for electrostatic
machines.

I use my old South Bend lathe, where I
adapted a tool rest made of a vertical steel bar fixed by two
nuts in a general-purpose support that adapts to the carriage of
the lathe, with a wood cylinder that can rotate around it. I made
also a live center for the tailstock, and a spinning tool, that
is a long steel bar with one end cut to a wedge shape, with well
rounded borders and polished to a mirror finish, and a wood
handle. The wood cylinder serves as rest for the tool, and can
rotate with it when under pressure. The vertical bar serves as
fulcrum for the lever movements of the tool. I found this
disposition very convenient.

The form, or chuck, is cut in hard wood, with a ball turning
tool for precise finishing, and has the shape of a little more
than half ball. It is held by the 3-jaws chuck of the lathe, well
centered. Note that this is rather dangerous because the tool can
escape and hits the jaws. Better would be to fix it to a face
plate. The form can be used many times, to produce identical
pieces.

A metal disk, in this case 0.4 mm copper, is cut with metal
scissors, annealed by heating to a dark red color and fast
cooling by immersion in water, and mounted between the form and
the live center in the tailstock. A wood block with a concave
form approximately matching the curvature of the form is used to
press the disk against the form.

Some paraffin (a candle) is rubbed against the disk and the
tip of the tool, and the spinning process is started. I set the
lathe speed to a medium value, and gently start to press the disk
against the form using the steel bar.

The center is pressed initially, while keeping the outer
border of the disk flat. This firmly presses the metal against
the form, and allows safe pressing of the outer areas of the disk
into a bell shape. The procedure is to produce a shallow
depression with the tool tip and to move it by slowly moving the
tool tip from the center to the edge of the disk. Some reverse
movements can also be made, to avoid excessive stretching of the
metal.

Care must be taken to avoid the formation of ripples, that can
cause the tearing of the metal. If they form, due to excessive
pressure applied, the disk must be removed from the lathe,
possibly annealed, and the ripples must be hammered out between
wood blocks.

When the half-ball is almost complete, it is necessary to cut
out the excess of material, with metal scissors (I found that to
try to use a cutting tool in the lathe for this is an invitation
to disaster), and anneal the metal again. The next operation is
to press completely the disk against the form, compressing the
outer border. At this point is not uncommon a rupture of the
metal, so great care must be taken. As the form curvature goes a
bit after 180 degrees, the metal gets firmly fixed to the form
(if it isn't, anneal and press it again). The tailstock can then
be moved away and the half-ball polished using the flat surface
of the tool, smoothing any surface irregularity, including the
area that was covered by the tailstock pressure block. I do this
running the lathe at high speed and with good lubrication
(paraffin), if necessary checking by touch if there is any
irregularity in the metal surface.

The final operation is to force the half-ball out of the form
(simple, with a lever using the vertical bar and a screwdriver)
and trim it to the exact height, by sanding the border with a
sandpaper foil over a flat surface, with frequent measurements of
the height, that shall be of exactly half diameter.

With some care, the result is a half-ball that will require
very little further polishing. Two halves are made and glued with
epoxi glue applied to the interior of the borders. The outer side
is kept clean of glue, as it is difficult to remove it later. At
a hole at the center of one side, a screw is glued with the
thread outside for fixation. Brass or copper balls can also have
the two halves soldered together. I apply a small amount of
solder to the internal edges of the halves, mount the ball in the
lathe lightly pressed between wood blocks, and apply a large
soldering iron to the joint, while rotating the ball slowly. When
some molten solder start to leak, the soldering is done. It's
better to let the smallest possible amount of solder to leak, as
it's difficult to remove it from the outer surface. A fixation
screw can be soldered too, before the operation. In any case, if
the halves get misaligned, heat from an alcohol lamp opens
the union, that can be then tried again. Don't try to correct
large misalignments by grinding.

Below, two half-balls made as shown, and some other half-balls
of several sizes and materials (aluminum, copper, and brass), as
they were obtained from the same spinning proccess. Note that
brass is harder, and requires more annealings. The final
polishing will be done only after the halves are joined.

The process is not fast, and rather difficult to master, but
making solid balls is not easy or fast too. These hollow balls
are much lighter, and use much less material. Similar procedures
can be used to make round cylinder ends, disks with rounded
borders, and even toroids.

See my links about electrostatic
machines, metalworking, and other subjects